This invention pertains to bridge circuits, having bipolar source drivers, intended for driving inductive loads, and more particularly to such a bridge circuit wherein each source driver has an associated auxiliary bipolar transistor for when the base-collector junction of a source transistor is momentarily forward biassed, preventing charge storage in the source transistor, and thus preventing transient shoot-through currents.
Referring to FIG. 1, a conventional full-wave bridge circuit has two NPN source drivers 12 and 13 having their collectors connected to a positive DC supply conductor 11. Two NPN sink drivers 16 and 17 are also provided having their emitters connected to a circuit ground conductor 21, which is normally connected to the circuit ground either directly or otherwise through a switch, a current sensing resistor, etc. Bridge-control input conductors 14, 15, 18 and 19 are connected to the bases of the driver transistors. An inductive load 20 is connected between the emitters of the source drivers 12 and 13 as well as being connected between the collectors of the sink drivers 16 and 17.
In a bridge driver which has an inductive motor load and is operated in a pulse width modulated (PWM) mode, there is a transient shoot-through current condition which can occur when a bipolar bridge sink driver transistor turns on. For example, inductive load 20 may be a motor, and during sink side PWM control of the motor 20, signals are applied to the four bases of the four transistor drivers so that drivers 12 and 17 are turned off, and driver 13 is turned on, all during the time while driver 16 is pulse width modulated (PWM) on and off at a duty factor to drive the motor 20 at a desired speed. With driver 12 off and driver 16 operating in the PWM chop mode, the inductive load terminal 22 will be connected (via the on driver 13) continuously to the positive DC supply voltage, Vbb, while the inductive load terminal 24 will alternate between ground voltage (zero) plus Vsat of the driver 16 (e.g. 0.1 volt) and a voltage more positive than Vbb which is attributable to the inductor 20 causing the current which was formerly flowing in driver 16 to flow through the forward biased diode 26.
Thus each time the sink driver turns off, the motor terminal 24 becomes greater than Vbb and the protective diode 26 conducts. At this time, the base-collector junction of the source driver 12 will also become forward biased, because the base of source driver 12 is connected via resistor 28 to motor terminal 24. This later condition results in the driver transistor 12 turning on in the inverse direction which results in the storage of charge in the base of driver 12. If the pulse width modulated sink driver 16 turns on while diode 26 conducts, then the stored charge causes the driver 12 to turn on transiently in the forward direction. Thus a large transient current may be produced between Vbb and ground through turned-on driver 16 and momentarily turned-on driver 12. This transient shoot-through current occurs repeatedly during the PWM operation of sink driver 16, and results in greater demands on the power supply and in the case that the bridge driver is in an integrated circuit, produces unwanted concentrations of heat there.
In the patent to Alberto Bilotti, U.S. Pat. No. 5,075,568 issued Dec. 24, 1991 and assigned to the same assignee as is the present invention, there is described a protective circuit for clamping the bases of source driver transistors to prevent voltages applied thereto from rising above the supply voltage Vbb, and thus preventing the above-described unwanted shoot-through currents. During those moments when the diode 26 is forward biased by a voltage V.sub.26, the clamping current is driven by the inductive load 20 from the supply voltage Vbb, through the turned-on driver 13, the load 20, the resistor 28 and then through a clamping circuit to ground. This clamping circuit holds the base of transistor 12 at a potential approximately equal to Vbb, thereby preventing the base collector junction of transistor 12 from forward biassing. This causes a power loss, P.sub.clamp, from the current in the resistor 28 such that ##EQU1##
In situations wherein Vbb is large and the resistance R.sub.28 of resistor 28 is small, this additional power loss and commensurate heating in an integrated circuit can be significant when the bridge is used for driving an inductive load in a micro stepping mode.